Known radiotelephony systems such as G.S.M are dedicated essentially to voice communications. They use a channel comprising two symmetrical links: a downlink (from a land base station to a mobile station) and an uplink (from the mobile station to the base station).
Systems currently being developed are also based on such a structure. Thus, the UMTS standard defined by ETSI allows for symmetric distribution between the downlink and the uplink.
It has also been proposed to extend the radiotelephony system by adding at least one supplementary channel to the main channel, in the down direction only and dedicated to high speed data transmission, such as files transmitted on the Internet network.
In the framework of this invention, it is assumed that the radiotelephony system is of the type comprising a symmetric two directional main channel and at least one supplementary channel as mentioned above.
It is also assumed that the supplementary channel uses a multi-carrier technique for distribution of data in the time/frequency space and presents a structure in predefined entities each including a predetermined number of symbols. It should be understood that the predefined entities are frames or sub-frames.
In order to simplify the problem, the following contains a presentation of the disadvantages of prior art with regard to the following particular application: synchronisation at sub-frame level of a supplementary OFDM HS-DPA link associated with a UMTS main channel. However, it is clear that this discussion may be transposed to other radiotelephony systems comprising a symmetric two-directional main channel and at least one supplementary channel. This discussion may also be transposed to synchronisation at the frame level of the supplementary channel.
Note that the HS-DPA (High Speed Downlink Packet Access) supplementary channel is a high-speed downlink associated with the UMTS main channel. Its objective is to increase the downlink speed to offer services requiring high speed (multimedia, video streaming, etc.).
As illustrated in FIG. 1, the UMTS main channel has a structure organised into N, N+1 frames each including 15 slots (time intervals) S1 to S15. As illustrated in FIG. 2, the HS-DPA supplementary channel has a structure organised into N, N+1 frames each including up to 5 sub-frames SF1 to SF5. Furthermore, each slot or sub-frame comprises a set of symbols, and each symbol comprises a set of signal units (chips).
Two technical solutions are proposed for the physical layer of the HS-DPA supplementary channel:                a spectrum spreading system complying with the UMTS system;        a system based on an OFDM multi-carrier modulation.        
With the first solution, a supplementary UMTS HS-DPA link is obtained that is inherent to the UMTS system. Therefore, it can benefit from all techniques already used by the UMTS main channel such as the channel estimate, control of power and clocks, and particularly synchronisation done with the CPICH signal specified in UMTS standard.
With the second solution, a supplementary OFDM HS-DPA link is obtained that uses a modulation different from that used in the UMTS system (spectrum spreading, CDMA). Consequently, it cannot use all techniques used in the UMTS system. Therefore, it must use specific techniques so as to perform the same functions. Nevertheless, some adaptation to the context may facilitate setting up and maintenance of communication on the OFDM link.
An OFDM sub-frame of the supplementary OFDM HS-DPA link and a UMTS sub-frame of the UMTS HS-DPA UMTS supplementary channel have the same duration (namely 2 ms).
Synchronisation in time is one of the key elements in setting up a communication. This synchronisation is broken down in several “layers” due to the nature of the radio-mobile cellular communication system that defines the two entities: sub-frame and frame (see FIG. 2). Thus, for the HS-DPA supplementary channel, this synchronisation is divided into several steps:                synchronisation at chip level, that consists of searching for the position of symbols (and therefore chips included in these symbols) depending on the clock used;        synchronisation at sub-frame level, that consists of searching for the beginning of sub-frames;        synchronisation at frame level, that consists of searching for the beginning of each frame.        
The UMTS HS-DPA supplementary channel may be synchronized relatively easily. As indicated above, because the UMTS HS-DPA supplementary channel is intimately linked to the UMTS system, its synchronisation may be based directly on synchronisation of the UMTS main channel. Thus, the initial synchronisation of the UMTS HS-DPA supplementary channel at chip level may be synchronised by a time self-correlation on a specific synchronisation signal (PSCH) forseen in the UMTS. After this synchronisation has been acquired at chip level, the synchronisation of the UMTS HS-DPA supplementary channel at sub-frame level can be done by searching for the beginning of UMTS slots (knowing that each sub-frame contains a predetermined number of UMTS slots, for example 3). This search is made using the PSCH signal. This signal is in the form of a packet of 256 identical chips emitted at the beginning of each slot. Finally, the frame synchronisation of the UMTS HS-DPA supplementary channel is done using the SSCH (Secondary Synchronisation Channel) signal that has the same shape as the PSCH signal except that the transmitted packets of 256 chips are modulated by known information. FIG. 3 illustrates the order of the different synchronisation steps of the UMTS HS-DPA supplementary channel at chip, slot and frame levels respectively.
On the other hand, at the moment synchronisation of the OFDM HS-DPA supplementary channel is more difficult because this link is not intimately linked to the UMTS system, unlike the UMTS HS-DPA supplementary channel.
According to current practice, the OFDM HS-DPA supplementary channel may be synchronised at chip level using the guard interval that represents part of the OFDM symbol (the last part). This synchronisation at chip level is obtained by a simple classical self-correlation on the received OFDM HS-DPA signal.
But once this synchronisation has been obtained at chip level, it is impossible to know the beginning of sub-frames and frames because the OFDM signal specified for the HS-DPA does not contain the PSCH and SSCH signals necessary for synchronisations at sub-frame and frame levels.
According to current practice, synchronisation of the OFDM HS-DPA supplementary channel at sub-frame level is specific to the OFDM system and is based on insertion of specific signals (in the form of OFDM symbols) to synchronisation at sub-frame level.
The major disadvantage of current practice is that the above mentioned specific synchronisation signals increase the load on the OFDM HS-DPA supplementary channel, or if the load is fixed, lower the useful speed.